Field of the invention.
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The present invention relates to a composite bead assembly comprising
a number of windings of a single elongated element.
Background of the invention.
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Bead assemblies or bead structures are generally known in the art and
are present in the majority of rubber tires. Their function is to anchor the
tire on the rim and to secure the transfer of forces and moments
between the tire and the rim.
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Bead assemblies comprising a number of windings of a single elongated
element are also known : a single elongated element forms several
windings, one winding adjacent to another winding until a first layer is
formed ; thereafter the same single elongated element forms in several
windings a second layer above the first layer, and so on. Such a bead
assembly can be designated as a 1xn bead assembly, where n refers to
the number of windings.
Other bead assemblies are formed by more than one elongated
element. One elongated element forms a first layer in several windings.
The second layer is formed by the windings of a second elongated
element, and so on. Such a bead assembly can be designated as a
nxm bead assembly, where m refers to the number of layers and n to
the number of windings in one layer.
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In a world-wide trend to continuously reduce consumption of fuels of
vehicles, numerous measures have been taken to reduce the weight of
the vehicles. The tires of the vehicles did not escape from this trend. In
the beginning the efforts were focused on the breaker and carcass plies.
In a second stage, however, the bead area was also scrutinized. With
respect to the weight reduction in the bead area, the ultimate target
seems to have been reached by a bead assembly where the strength
members are formed by high-tensile synthetic filaments or synthetic
cords such as aramid. A bead assembly comprising aramid filaments is
mentioned in EP-A-0 416 638. A bead assembly comprising aramid
cords is mentioned in DE 197 06 262. A part from the problem of creep
experienced with aramid strength members, all bead assemblies with
aramid strength elements have the major drawback of being very
expensive : they cost up to seven and more times the price of a
conventional bead assembly with strength members in steel.
Summary of the invention.
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It is an object of the present invention to provide a bead assembly which
realizes a considerable weight reduction without multiplying a number of
times the price of a convenient bead assembly.
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According to the invention there is provided a composite bead assembly
comprising a number of windings of a single elongated element. This
single elongated element comprises a steel core and a coating of matrix
material around the steel core. This matrix material around the steel
core is softer than the steel core and, as a consequence, helps to
distribute the tensions uniformly over the steel cores and avoids the use
of mechanical clamps to hold the windings together. The steel core
comprises one or more steel filaments. The steel filaments have a
diameter d ranging from 0.10 mm to 0.60 mm, e.g. from 0.10 mm to
0.30 mm. These are diameters which are convenient for reinforcement
of the breaker plies and, at least for the lower half between 0.10 mm
and 0.30 mm, for the reinforcement of carcass plies of a rubber tire, but
which are not convenient for the reinforcement of the bead area. The
steel filaments of the invention have a tensile strength more than
4000 - 2000 x d (MPa)
wherein 1 MPa = 1 MegaPascal = 1 N/mm2 and wherein d is the
diameter of the steel filament expressed in mm.
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A steel filament having a tensile strength above the level expressed in
formula (1) is hereinafter referred to as an "ultra high-tensile" steel
filament.
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The individual steel filaments may be subjected to a stress-relieving
treatment carried out in such a way that an increased elongation at
break is obtained or may be not subjected to such a stress-relieving
treatment. Within the context of the present invention, preference is
given to steel filaments which are not subjected to a stress-relieving
treatment, since the stress-relieving treatment considerably reduces the
tensile strength and it would become difficult to obtain with stress-relieved
ultra high-tensile strengths as expressed by the level of formula
(1).
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With respect to the steel core, two embodiments are possible.
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In a first embodiment the steel core is a bundle of steel filaments, i.e.
two or more individual steel filaments in parallel adjacent to each other.
The number of windings depends upon the number of filaments in the
bundle and upon the diameter of the filaments. The greater the number
of filaments and the greater the diameter of the filaments, the smaller
the number of windings. In any case, the number of windings exceeds
3.
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In a second embodiment the steel core is a single steel filament. In
order to obtain the required breaking load, the number of windings is
greater than 3, but very often exceeds 100 and in some embodiments
even 200.
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With respect to the matrix material of the coating around the steel core,
this can be a rubber, a synthetic rubber or a natural rubber or a high
polymer. A high polymer is herein defined as an organic macromolecule
composed of a large number of monomers so that the molecular weight
exceeds 5000. Preferably the high polymer has a melting point or
melting zone above 180 °C, or most preferably above 190 °C. The high
polymer is preferably selected from polyamids, polyurethanes or
polyester based thermoplastic elastomers.
The thickness of the coating is preferably smaller than or equal to
0.15 mm.
Brief description of the drawings.
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The invention will now be described into more detail with reference to
the accompanying drawings wherein
- FIGURE 1 shows a transversal cross-section of a first example
of a bead assembly ;
- FIGURE 2 shows a detailed view of a transversal cross-section
of an elongated element of a bead assembly ;
- FIGURE 3 shows a transversal cross-section of another example
of a bead assembly and a detailed view of a transversal cross-section of
an elongated element of this bead assembly ;
- FIGURE 4, FIGURE 5 and FIGURE 6 all show different
geometrical cross-sectional configurations of bead assemblies.
Description of the preferred embodiments of the invention.
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FIGURE 1 shows a transversal cross-section of a first example of a
bead assembly 10 according to the invention. The bead assembly 10
comprises 220 windings of a single elongated element 12 : twenty
windings one next to the other in a horizontal direction and forming one
layer, and eleven layers of twenty windings, one layer above the other
layer.
Such a single elongated element 12 is shown in detail in FIGURE 2.
The elongated element 12 comprises an ultra high-tensile steel filament
with a diameter of 0.18 mm as core 14. The steel filament 14 has a
coating 16 of a high polymer material. The global diameter of the
elongated element 12 is 0.22 mm. The total length of the elongated
element 12 in the bead assembly is about 267 m for an internal diameter
of the bead assembly of 384 mm which fits for a passenger car tire with
a rim diameter of 15 inch.
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FIGURE 3 shows a transversal cross-section of a second example of a
bead assembly 18 according to the invention. The bead assembly 18
comprises 24 windings of a single elongated element 19 : six windings
one next to the other in a horizontal direction and forming one layer, four
layers of six windings, one layer above the other layer.
The single elongated element 19 comprises a bundle 20 of seven
individual filaments 21 and a coating 22 of a high polymer material
around the bundle 20. The seven individual steel filaments 21 of the
bundle 20 all have a diameter of 0.20 mm and all have an ultra high-tensile
strength. The diameter of the non-coated bundle 20 is 0.60 mm
(= 3x0.20 mm) and the diameter of the coated elongated element 19 is
0.64 mm. The breaking load of an individual steel filament is about
121 N.
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The bead assemblies of FIGURE 1 and FIGURE 3 have a rectangular
transversal cross-section. The technique of winding a single elongated
element comprising a steel core and a synthetic coating is, however, not
limited to rectangular cross-sections. Other cross-sections are possible.
Some of these cross-sections are illustrated in FIGURE 4, FIGURE 5
and FIGURE 6.
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FIGURE 4 shows a bead assembly 24 where the windings of the
elongated element 25 form a hexagonal cross-section.
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FIGURE 5 shows a bead assembly 26 where the windings of the
elongated element 27 form a triangular cross-section.
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FIGURE 6 shows a bead assembly 28 where the windings of the
elongated element 29 form a trapezium cross-section.
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A bead assembly according to the invention has been compared with
two existing bead assemblies with regard to weight and with regard to
price. Table 1 hereunder summarizes the results.
| Type of bead assembly |
| | number of windings | length of element (m) | weight (%) | price/bead (%) |
| 0.89 mm steel wire single element 1x16 | 16 | 19.5 | 100.0 Ref. | 100 Ref. |
| aramid filament 3360 dtex | 44 | 53.2 | 33.9 | ±750 |
| invention : 1x216 0.18 mm ultra-high tensile steel core, coated to diameter of 0.22 mm | 216 | 267 | 56.5 | ±150 |
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The values of weight and price in the above Table 1 are relative values.
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A tensile test has also be carried out on different bead assemblies.
Table 2 hereunder summarizes the results.
| Type of bead assembly | Breaking load bead (KN) | Reference elongation (%) | Coefficient C (%) |
| 1 | 34.1 | 3.35 | unknown |
| invention 2 | 37.7 | 2.66 | 95 |
| 3 | 33.9 | 2.14 | 81 |
| 4 | 36.4 | 2.13 | 87 |
| 5 | 37.5 | 2.54 | 84 |
| 6 | 30.6 | 2.21 | 69 |
| 7 | 33.3 | 2.06 | 75 |
| 8 | 29.7 | 2.18 | 68 |
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In the above Table 2, the numbers in the first column refer to following
types of bead assemblies :
- 1 : Aramide composite bead 15 inch ;
- 2 : invention 1x216 bead assembly, 0.18 mm ultra high-tensile filament,
coated with high polymer until diameter of 0.22 mm, 216 windings ;
- 3 : 0.89 mm steel wire ; 4x4 bead construction, thickness of layer
1.2 mm ;
- 4 : 0.89 mm steel wire ; 1x16 bead construction, diameter of extruded
wire 1.1 mm ;
- 5 : 0.86 mm high-tensile steel wire ; 4x4 bead construction, stress
relieved to obtain higher elongation, thickness of layer 1.2 mm ;
- 6 : 0.825 mm high-tensile steel wire ; 4x4 bead construction, not stress
relieved, thickness of layer 1.2 mm ;
- 7 : 0.825 mm high-tensile steel wire ; 1x16 bead construction, not stress
relieved, extruded diameter 1.03 mm ;
- 8 : 3x0.20+6x0.35 high-tensile cord ; 3x3+2 bead construction.
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The "Reference Elongation" is the elongation at fracture of a bead which
is uniformly elongated over the total bead circumference so that there
are no local deformations.
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The bead assembly according to the invention has the greatest value in
Reference Elongation.
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The "Coefficient" C is the measured breaking load FBm divided by the
theoretical breaking load. The theoretical breaking load is equal to twice
the number n of wires per bead multiplied by the breaking load per wire
FBW. The Coefficient is expressed in per cent.
C = 100 x FBm / 2 x n x FBw (%)
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The bead assembly according to the invention has also the greatest
value in Coefficient.
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Other advantages of the bead assembly according to the invention, in
comparison with conventional beads are a better stress distribution over
the cross-section of the bead due to the matrix material and due to the
high number of windings, an improved efficiency, a better adaptability to
the tire sizes, a higher uniformity and a reduced cross-sectional surface.
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A steel composition used for the ultra high-tensile steel filaments is
along following lines : a carbon content ranging from 0.85 % to 1.0 %, a
silicon content ranging from 0.10 % to 1.0 %, a manganese content
ranging from 0.10 % to 0.60 %, a chromium content ranging from 0.05
to 0.40 %, a maximum phosphorous content of 0.05 %, a maximum
sulfur content of 0.05 %, the remainder being iron.
As a matter of example, the carbon content is 0.92 %, the manganese
content 0.30 %, the chromium content 0.20 %, and the silicon content
0.20 %.
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The ultra high-tensile steel filaments may be coated with bronze, zinc,
brass, copper or a particular coating may be absent. The type of
coating mainly depends upon the type of matrix material to be provided
around the steel filament. Preference is given to coatings that enable an
easy manufacturing (e.g. brass, copper and bronze which facilitate the
hard drawing steps) or to coatings that improve the adhesion between
the steel filaments and the matrix material.
